FAN2011 Datasheet, PDF (7/10 Pages) Fairchild Semiconductor – 1.5A Low Voltage Current Mode Synchronous PWM Buck Regulator

Block Diagram
EN
V_IN
DIGITAL
SOFT START
UNDER VOLTAGE
LOCKOUT
IS
CURRENT
SENSE
FB
ERROR
AMP
COMP
LOGIC
CONTROL
MOSFET
DRIVER
SW
0.8V
GND
IS
OSC
SLOPE COMPENSATION
OVER
VOLTAGE
COMP
REF FB
GND
Figure 4. Block Diagram
Detailed Operation Description
The FAN2011 is a step-down pulse width modulated (PWM)
current mode converter with a typical switching frequency of 1.3
MHz. At the beginning of each clock cycle, the P-channel tran-
sistor is turned on. The inductor current ramps up and is moni-
tored via an internal circuit. The P-channel switch is turned off
when the sensed current causes the PWM comparator to trip
when the output voltage is in regulation or when the inductor
current reaches the current limit (set internally to typically
2600mA). After a minimum dead time, the N-channel transistor
is turned on and the inductor current ramps down. As the clock
cycle is completed, the N-channel switch is turned off and the
next clock cycle starts. The duty cycle is solely given by the ratio
of output voltage and input voltage. Therefore, the converter
runs with a minimum duty cycle when output voltage is at mini-
mum and input voltage is at maximum.
100% Duty Cycle Operation
As the input voltage approaches the output voltage and the duty
cycle exceeds the typical 95%, the converter turns the P-chan-
nel transistor continuously on. In this mode the output voltage is
equal to the input voltage minus the voltage drop across the P-
channel transistor:
V_OUT = V_IN – I_LOAD × (R_{DS_ON} + R_L), where
R_{DS_ON} = P-channel switch ON resistance
I_LOAD = Output current
R_L = Inductor DC resistance
UVLO and Soft Start
The reference and the circuit remain reset until the V_IN crosses
its UVLO threshold.
The FAN2011 has an internal soft-start circuit that limits the in-
rush current during start-up. This prevents possible voltage
drops of the input voltage and eliminates the output voltage
overshoot. The soft-start is implemented as a digital circuit
increasing the switch current in four steps to the P-channel cur-
rent limit (2600mA). Typical start-up time for a 40µF output
capacitor and a load current of 1500mA is 800µs.
Short Circuit Protection
The switch peak current is limited cycle by cycle to a typical
value of 2600mA. In the event of an output voltage short circuit,
the device operates with a frequency of 400kHz and minimum
duty cycle, therefore the average input current is typically
350mA.
Thermal Shutdown
When the die temperature exceeds 150°C, a reset occurs and
will remain in effect until the die cools to 130°C, at that time the
circuit will be allowed to restart.
Applications Information
Setting the Output Voltage
The internal reference is 0.8V. The output is divided down by a
voltage divider, R1 and R2 to the FB pin. The output voltage is:
V_OUT
=
V_RE
F


1
+
R^{-R----12-}
According to this equation, and assuming desired output volt-
age of 1.5096V, and given R2 = 10KΩ, the calculated value of
R1 is 8.87KΩ.
FAN2011/FAN2012 Rev. 1.0.2
7
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